1. Technical Field
The present invention relates to stand alone, digital electronic liquid measuring system for use on bulk liquid storage tanks. This general purpose device is designed to be easy to install and calibrate, intrinsically safe around flammable liquids, and is solar powered requiring no on site connections.
2. Background
A number of methods and apparatus for measuring the liquid level in a bulk storage tank exist. Most of the early methods utilized floats and levers in various combinations to move an indicator or pointer or balance beam. Newer electronic methods ranging from simple switches and resistors to more complicated systems of ultrasonic or microwave transducers are also used depending on the specific application. Most of these systems are not capable of high accuracy and may be difficult to install and maintain. On the other end of the list are monitoring systems that utilize precision load cells and displacement methods that can be calibrated for high accuracy. These are most often used in leak detection systems of underground storage tanks in the petroleum industry such as U.S. Pat. No. 5,132,923 issued to Crawford et al.
The primary problem with all of these systems is that they are either too accurate and expensive for general applications, requiring special adaptation to each installation, or they are not accurate enough, difficult to read and difficult to maintain. In addition there are no digital electronic gages that are solar powered and can operate without external power connections.
Most of the patents that describe measuring a liquid in a tank, by measuring the buoyancy force acting on a displacer, are similar to the one described in U.S. Pat. No. 4,945,756 issued to Lewis et al. These devices all utilize a load cell as the force measuring device in connection with precision Wheatstone Bridge circuitry. This method is very accurate but requires a large amount of electronics to operate the load cell. Load cells can be easily damaged, and tend to be rather expensive.
A different method other than the use of a load cell is disclosed in U.S. Pat. No. 4,300,388 issued to Hansel et al. This requires use of at least two fixed coil windings working in relation to a moveable ferrite core. Another similar patent, U.S. Pat. No. 4,537,061 issued to Demeyer et al., discloses a device that measures liquid level by measuring the buoyancy force acting on a displacer. The weight of the displacer is measured by the movement of a ferrite core attached to the displacer that is supported by a spring. The spring offsets the weight of the displacer and allows the ferrite core to move in or out of the variable transformer. This method differs from the present invention in two important respects. The variable transformer must use at least two or more windings and depends upon magnetic coupling of the primary and secondary transformer windings by the ferrite core. The other important difference is that the amplitude of the AC voltage on the secondary transformer is what is measured, not the frequency. This AC output is then converted to a dc signal that is further processed in analog format.
Some of the goals of this invention include: providing a low cost digital indicator that can be calibrated to read volume, level, or weight of a liquid; an indicator which self contained and self powered; and an indicator which is easy to install and set up in a variety of tank sizes and shapes. An additional goal of the invention is to provide a high level indicator to prevent over filling of the tank.